Up to now detonators of the above-mentioned type in commercial use are generally represented by pyrotechnic delay detonators which contain a small charge of a primary explosive placed in contact on one side with a pyrotechnic delay charge and on the other with a secondary explosive base charge, to effect the transition from a relatively slow non-violent chemical burning of the delay charge initiated by an igniting means such as an electrical fuse head to a detonation in said base charge.
In this connection it should be noted that for practical purposes a primary explosive is defined as an explosive substance which can develop complete detonation from a flame or a conductive heating within a volume of a few cubic millimeters of the substance, even without any confinement thereof. On the contrary, however, a secondary explosive can be initiated to detonate by a flame or a conductive heating only if present in very much larger quantities or within heavy confinement such as a heavy walled metal container, or by being exposed to mechanical impact between two hard metal surfaces. Examples of primary explosives are mercury fulminate, lead styphnate, lead azide and diazodinitrophenol or mixtures of two or mor eof these and/or other similar substances. Representative examples of secondary explosives are pentaerythritoltetranitrate (PETN), cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMZ), trinitrophenylmethylnitramine (Tetryl) and trinitrotoluene (TNT) or mixtures of two or more of these and/or other similar substances.
In a widely use method of producing a pyrotechnic delay detonator according to the prior art the required weight of secondary explosive for the base charge, typically about 600 mg, is first pressed into the bottom part of a outer metal shell having a closed bottom end. The required weight of primary explosive, typically about 300 mg or less, is then loosely filled into the shell on top of the base charge and compacted by pressing into the shell. Said primary explosive also contains a previously compacted pyrotechnic charge which is thus left with its upper end exposed and its lower end in close contact with the compacted primary explosive.
When exposed to an igniting means such as a flame from an electric fusehead, from a NONEL.RTM. tube or from a detonating cord, inserted into the open end of the detonator shell, the pyrotechnic charge starts burning at a rate that is typically of the order of 2-10 cm/s. As soon as the burning pyrotechnic charge reaches the primary explosive there is a rapid transition from burning to detonation within said primary explosive. The resulting detonation in turn initiates detonation in the secondary explosive base charge.
Conventional detonators of the above type have several serious disadvantages. These are primarily derived from the extreme sensitivity of the primary explosive to impact, friction or flame initiation. Some of said disadvantages are:
1. The presence of even a small charge of primary explosive makes a conventional detonator hazardous to handle because it is sensitive to mechanical deformation or impact, such as when accidentally bent or impacted at the region of the primary charge.
2. The manufacture of the primary explosive, the handling of the same and the shell filling operations during manufacture of the detonator are risky operations which require extreme care and caution which in turn means costly operations and expensive plants.
3. The drainage of poisonous water containing fulminate, lead or phenol from the process of manufacture of the primary explosive creates serious environmental pollutions if not controlled. Furthermore, as it is not allowed to transport the bulk primary explosive it is necessary for each detonator plant to set up its own primary explosive plant, which increases the number of polluted areas and also requires additional investments for environmental protection.
It is therefore an object of the present invention to eliminate or minimize the above-mentioned risks, pollutions and high investments, which is accomplished by using instead of the primary explosive a secondary explosive as an initiating charge for detonators of the type referred to above.
It is true that more recently non-primary explosive detonators have been disclosed and patented but generally these new detonators have not come to any widespread general use due to certain disadvantages or limitations. Among such disadvantages and limitations are that generally these previously known non-primary explosive detonators are restricted with reference to the use of the igniting means and with reference to the wall thicknesses of the detonator shell and the dimensions of the confinement for the secondary explosive initiating charge and that they are also generally relatively complex as to their structures which influences upon the manufacture as well as the operation thereof.
As prior art related to non-primary explosive detonators or in connection therewith reference is made to the following patent specifications:
U.S. Pat. No. 3,212,439 discloses a blasting cap which contains secondary explosives only. The detonation of the secondary explosive is caused by another secondary explosive that is compressed and arranged in a confined enclosure in a steel tube having specific dimensions. This confined enclosure provides conditions under which an electrical ignitor ignites the secondary explosive.
U.S. Pat. No. 3,978,791 relates to a detonator device containing secondary explosives only. Also in this case a compressed secondary explosive, "donor secondary explosive", is utilized but together with an impactor disc, a portion of which is released and accelerated when said donor secondary explosive is initiated by a bridge wire. The disc strikes an acceptor secondary explosive with sufficient velocity to produce detonation of the acceptor secondary explosive.
U.S. Pat. No. 4,239,004 discloses a detonator device of a structure similar to that of U.S. Pat. No. 3,978,791 but the device also contains a delay mixture charge that imparts to the device a time delay before the donor secondary explosive is initiated.
DE AS No. 1 646 340 discloses a detonator device for the initiation of a non-sensitive explosive, which contains a fuse and a pyrotechnic time delay element and the essential feature of the device is that it comprises a housing filled with a secondary explosive and open at one end. The open end of the housing is facing the delay element of the other part of the device and removably attached thereto.
U.S. Pat. No. 3,724,383 (1973) relates to a new method of initiation of an explosive, viz. the use of a laser pulse that passes through a fiber optic bundle (9) and a focusing bead (4) to impinge upon a charge (11) of a secondary explosive which is set into low order detonation. A second secondary charge (10) is thereby set into low order detonation but as said second charge is loaded in a gradient of increasing density the velocity of the reaction increases very rapidly and a high order explosion is obtained.
U.S. Pat. No. 4,206,705 (1980) relates to an electrical initiator wherein polymeric solid sulfur nitride (SN).sub.x is utilized as the sole explosive initiating means thanks to its ability to act as an explosive as well as to conduct electrical current.
U.S. Pat. No. 3,661,085 discloses a new electric initiator structure wherein the pyrotechnic or explosive mix contacts only selected portions of the bridge wire, which means a substantially faster response time than that exhibited by conventional initiators. The explosive charges (primary and secondary charges) are those charges which are conventionally employed in such devices.